This application is based on application No. H11-202021 filed in Japan, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention pertains to a fusing device used in a copier, printer or facsimile machine based on the electrophotographic method, for example, and more particularly to a fusing device that fuses a toner image onto a recording medium by means of inductive heating.
2. Description of the Related Art
A fusing device that fuses onto a sheet of paper or transfer medium comprising a recording medium a toner image held on them is mounted in an image forming apparatus such as a copier, printer or facsimile machine that uses the electrophotographic method.
Various methods are used by the fusing device, but in response to the recent demand for low energy consumption, fusing devices employing the inductive heating method, which offers a higher conversion efficiency than that available in devices that use a halogen lamp as the heat source, have been proposed.
Fusing devices that use the inductive heating method have, as disclosed in Japanese Laid-Open Patent Application Hei 10-207265, a fixed or movable hollow conductive member, a closed magnetic circuit iron core, part of which is inserted into the empty space of the conductive member and which forms a closed magnetic circuit, and an inductive coil that is wrapped around the closed magnetic circuit iron core, such that the flow of electric current in the inductive coil generates a magnetic flux that gives rise to an inductive current that flows around the circumference of the conductive member, thereby inductively heating the conductive member.
However, fusing devices using this type of inductive heating method have the problem that the closed magnetic circuit iron core inside them requires that space for their mounting be created inside the device, making them large in size. In addition, because the closed magnetic circuit iron core is heavier than a halogen lamp, the member used to hold it must be sufficiently rigid to hold this weight, making the device more expensive.
Further, roller-type fusing devices also have the problem that because the roller diameter increases to accommodate the cross-sectional area of the closed magnetic circuit iron core, the curvature of the roller is large, and it consequently becomes difficult to separate the paper from the roller.
The object of the present invention is to provide an inductive thermal fusing device in which the fusing device itself may be made more compact and lightweight by reducing the size of the closed magnetic circuit iron core, and in which the separability of the paper based on the roller curvature may be improved where rollers are used.
These and other objects are attained by means of an inductive thermal fusing device comprising:
a cylindrical conductive member for applying heat to a recording medium holding toner;
an iron core which is inserted into the empty space inside the conductive member;
a coil wrapped around the iron core and along the length of the empty space inside the conductive member;
a power supply for supplying AC voltage to the coil; and
a modulator for changing the frequency of the AC voltage provided by the power supply.
These and other objects are also attained by the inductive thermal fusing device described above, said device further comprising a pressure member for directly or indirectly pressing the recording medium onto the conductive member,
These and other objects are also attained by the inductive thermal fusing device described above, said device further comprising a voltage detection circuit for detecting the voltage provided to the coil by the power supply, wherein the modulator changes the frequency based on the results of the detection by the voltage detection circuit.
These and other objects are also attained by the inductive thermal fusing device described above, wherein the modulator further has solid state relays connected in a bridge fashion and a CPU for controlling their switching, the detection circuit has a voltage detection resistor in which a prescribed voltage level is set and an operational amplifier for comparing the voltage level set in the voltage detection resistor and the level of voltage provided and rectified by the power supply, and outputs the result of the comparison to the CPU, and the CPU controls the switching of the solid state relays based on this output.
These and other objects are attained by means of an image forming apparatus comprising:
a toner image forming unit for forming a toner image on a photoreceptor;
a transfer unit for transfering the toner image formed on the photoreceptor onto a recording medium;
a power supply for providing AC voltage;
a modulator for changing the frequency of the AC voltage provided by the power supply; and
a fusing unit for fusing the toner image transferred to the recording medium onto the recording medium, wherein the fusing unit comprise
a cylindrical conductive member for applying heat to a recording medium holding toner;
an iron core which is inserted into the empty space inside the conductive member;
a coil connected to the power supply and wrapped around the iron core and along the length of the empty space inside the conductive member; and
a pressure member for directly or indirectly pressing the recording medium onto the conductive member.
These and other objects are also attained by means of an inductive thermal fusing device comprising:
a fixed or movable hollow conductive member;
a pressure member for directly or indirectly pressing the recording medium holding toner onto the conductive member;
a closed magnetic circuit iron core, part of which is inserted into the empty space of the conductive member, and which forms a closed magnetic circuit:
an inductive coil that is wrapped around the closed magnetic circuit iron core, such that it is linked to the iron core, said coil generating a magnetic flux that gives rise to inductive current that flows around the circumference of the conductive member;
a power supply for providing AC voltage to the inductive coil; and
a modulator for modulating the frequency of the AC voltage provided to the inductive coil by the power supply.
These and other objects are also attained by the inductive thermal fusing device described above, wherein the modulator comprises a voltage detection circuit that detects the AC voltage provided to the inductive coil by the power supply and modulates the frequency of the voltage provided to the inductive coil by the power supply based on the voltage detected by the voltage detection circuit.
These and other objects are also attained by the inductive thermal fusing device described above, wherein the modulator comprises a current detection circuit that detects the electric current that flows in the inductive coil due to the power supply and modulates the frequency of the voltage provided to the inductive coil by the power supply based on the electric current detected by the current detection circuit.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings.